1. Field of the Invention
The present invention related to a three-wave shared antenna of a rod type used in an automobile wherein the three-wave shared antenna can transmit/receive a radio mobile telephone wave (hereinafter called an MT wave), and receive a broadcast wave of an amplitude modulation system (hereinafter called an AM wave) and a broadcast wave of a frequency modulation system (hereinafter called an FM wave).
2. Description of the Related Art
Conventionally, there was provided this type of a three-wave shared antenna comprising an MT wave element formed in a two-step collinear manner wherein a phasing coil is provided in an intermediate portion of a rod-type element and an electrical wave length is (3/8) .lambda.+(5/8).lambda. to adjust to the MT wave. By the use of this type of the antenna, transmission/reception of the MT wave can be performed with high sensitivity. However, since the length of the element is too short, the sensitivity of transmission/reception of the AM or FM wave is poor. In other words, it is normally necessary for the antenna element for the AM wave or FM wave to have its length of about 0.75 to 0.95 m. Therefore, the length of the above-mentioned three-wave shared antenna is 0.35 to 0.4 m and is too short as compared with the normal length of the antenna. Due to this, lack of sensitivity occurs. In general, in order to eliminate the above disadvantage, impedance of a feeder cable is made lower, and the FM wave is amplified by a booster, thereby compensating lack of sensitivity. However, since an active element is used to improve sensitivity of FM wave in the above-mentioned antenna using the booster, there is a limitation in its amplification degree. Therefore, if the amplification exceeds its limitation, there occur troubles in such as a cross modulation or an intermodulation. In addition, there are problems in that increment of station to station noise occurring in a space region between a certain station to a next station, and decrement of S/N ratio.
Recently, three-wave shared antennas, which are contrived not to generate the above-mentioned problems, have been developed. In these improved antennas, a conductive mast is connected to the top portion of an MT wave element, which is formed in a two-step collinear manner for the above-mentioned MT wave, through a trap element. The trap element comprises an inductance element and a capacitance element, which are connected in parallel to each other. AM and FM waves are allowed to pass through the trap element, but the trap element has an MT wave interruption function by which MT wave is prohibited from passing through the trap element by generating parallel resonance. The conductive mast is arranged to generate resonance of a 1/4 wavelength in the FM wave and be able to receive the AM wave.
The above-structured antenna operates in the two-step collinear manner in the transmission/reception of the MT wave. In the reception of the FM and AM waves, the entire length of the antenna element is used as an antenna function. Therefore, according to the above-structured antenna, not only the transmission/reception of the MT wave but also the reception of the FM and AM waves can be performed with relatively high sensitivity. However, it cannot be said that the above-structured antenna has a satisfactory characteristic. Particularly, since the length of the antenna element is short, a focusing point level of a gain characteristic against the electric filed strength of the antenna cannot be improved. As a result, S/N ratio of FM wave and that of AM wave are low.
Additionally, in an automobile antenna, it is required that the antenna is fixed to a car body to be slightly inclined backward in terms of the design and the object wherein aerodynamic noise resulting from the reception of wind pressure is reduced. However, since the above-mentioned three-wave shared antenna comprises an MT wave element having strong directivity, sensitivity of the transmission/reception of MT wave is abruptly lowered if the antenna is fixed to the car body to be slightly inclined backward.
As mentioned above, in the three-wave shared antenna wherein the conductive mast, which is used for receiving AM and FM waves, is connected to the top end portion of the MT wave element formed in the two-step collinear manner through the trap element, there is not provided the necessary and sufficient antenna characteristics. It can be considered that the antenna characteristic can be improved by making the length of the conductive mast to be connected to the top end portion of the MT wave element sufficiently larger. However, the conductive mast has its limit in that its length is made larger as explained later. Due to this, the antenna characteristic cannot be improved by making the length of the conductive mast longer.
More specifically, logically speaking, the length of the conductive mast may be set to an odd-numbered multiple of (1/4).lambda. when the wavelength of the electric wave having a frequency of the radio telephone band is .lambda.. However, in actuality, this is limited to about five time. In other words, if the length of the conductive mast is set to much larger (for example, about seven times, or nine times), a radiation pattern is disordered when the transmission/reception of the wave is performed in the radio telephone band, so that various troubles occur. Moreover, such conductive mast is limited in the manufacture and the structure. For example, in order to contain the antenna in the car body, it is necessary to limit the length of the lower portion, which is lower than the feeding portion of the antenna, so as to adjust to the inner space of the car body. In consideration of the above-mentioned point, the length of the conductive mast is limited to about three times of (1/4).lambda.. Therefore, the entire length of the above-structured antenna is about 0.5 to 0.6 m.
As compared the above-mentioned antenna with the antenna, which is used for AM and FM waves, having the length of 0.75 to 0.95 m, lack of sensitivity cannot be avoided. Moreover, a good S/N ratio cannot be obtained.
On the other hand, if the above-mentioned three-wave shared antenna is fixed to be slightly inclined to the back of the automobile, there is a case that the sensitivity of the transmission/reception of MT wave is abruptly lowered. Therefore, the antenna was not able to be fixed to the car body to be inclined backward.